We propose to investigate the nature of the remarkable Be star MWC922.
This object was observed at positions 7 arcsecs apart in our guaranteed
(MUIZON_MMDUSTY) and open time (RVOORS_BEDISKS) programmes with SWS in
AOT01 speed 2. Both low resolution spectra show an incredible richness of
solid state structure, both of oxygen-rich and of carbon-rich chemistry.
The SWS spectrum reveals strong UIR emission at 3.3, 6.2, 7.7, 8.6 and in
one case at 11.3 micron. These bands are usually attributed to  polycyclic
aromatic hydrocarbons (PAHs). In MWC922 the 8.6 micron band is very strong
while the 11.3 micron band is unusually weak. This is the only
circumstellar dust shell in which such strange band strength ratios are
seen. The spectrum longward of 15 microns is full of emission bands from
crystalline silicates. We have seen no other source in which these bands
are so bright (250 Jy at 34 microns)
cent, of the oxygen-rich dust component, must be crystalline. This is
highly unusual: only comet hale-bopp has such high ratio of crystalline to
amorphous silicates.
The evolutionary status of MWC922 is uncertain. It has been classified
as a massive post-main-sequence star and as pre-main-sequence object.
The ISO-SWS spectrum however shows a some similarity with that of
the Red Rectangle. In this object, which is an F-type  post-AGB star,
C-rich dust is mainly in an extended nebula while O-rich crystalline dust
is observed in an equatorial disk which surrounds the central binary
of the Red Rectangle. It is believed that in the equatorial disk of
this system grain coagulation and grain growth occur as in the disks
surrounding young pre-main-sequence stars.
The similarity between the ISO-SWS spectra of the Red Rectangle and of
MWC922 suggest that the Be star MWC922 may be the successor of the Red
Rectangle. ...(end of abstract in OBS_SUMMARY)